Signal controlled amplifier



May 18, 1943 LE ROY C. PASLAY SIGNAL CONTROLLED AMPLIFIER Filed Aug. 19,1937 2 Sheets-Sheet l b25 57 Tau,

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SIGNAL CONTROLLED AMPLIFIER Filed Aug. 19, 1937 2 Sheets-Sheet 2Patented May 18, 1943 SIGNAL CONTROLLED AMPLIFIER Le Roy 0. Paslay,Chicago, 111., assignor to National Geophysical Company, a corporationof Delaware Application August 19, 1937, Serial No. 159,994

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This invention relates to a signal controlled amplifier, and moreparticularly to an electronic amplifier adapted to maintain asubstantially constant output despite wide variations in the signalinput thereto.

One feature of this invention is that it is adapted to maintain asubstantially constant average output and yet reproduce individual inputoscil lations in proportion to those adjacent thereto; another featureof this invention is that it will maintain the average value of anoscillatory output substantially constant when the mean oscillatoryinput or signal varies uni-directionally through a wide range during ashort interval of time; yet another feature is that improved means isprovided for automatically varying the gain of the amplifier withrespect to time during a predetermined interval, independently of inputvariations but inversely thereto; yet another feature of this inventionis that it is adapted to regulate I While circuits for automaticallycontrolling the output of an amplifier to maintain itsubstantiallyconstant despite variations in the input are known and in common usage,these are found almost entirely in the radio field where the signal issuperimposed upon a carrier. The carrier,

as distinguished from the signal envelope, provides a plurality of rapidoscillations which can be rectified to provide a control voltage for oneof the elements of a tube having a variable ampliflcation factor, or aplurality of'such tubes, and thus control of the output is readily andsimply achieved. Where such a carrier is not present, however, and anattempt is made to control the output directly from the signal, variousdimculties are encountered with conventional automatic volume controlcircuits. This is particularly true where the signal frequency is ratherlow and where the variations in the input cover an exceedingly widerange in a short interval of time. In such a case conventional methodsof automatic volume control will, if arranged with a short enough timeconstant and a wide enough range to be of much value distort both thewave forms of individual waves and the amplitude relation betweensucceeding waves.

The present inventions-are disclosed in connection with an electronicamplifier particularly designed for use in geophysical exploration work.In such exploration an explosive charge is detonated at a particularpoint and the vibrations resulting therefrom and transmitted through theadjacent terrain are picked up at various points by seismometers and,after being amplified, recorded in some convenient way, as by agalvanometer on a photographic strip. The resulting record comprisesoscillations of low fr quency, and from the Wave form, time intervalsand other data reproduced on the record by the reflections of the samedetonation from various strata the geology of the terrain can bededuced.

The physical waves resulting from the detonation and subsequentlytransformed into electrical energy by the seismometers are of lowfrequency, generally less than 100 cycles per second; and in an intervalof five seconds or less the average or mean amplitude of the input tothe amplifier decreases to one ten-thousandth or less of the initialinput. This variation in mean signal strength may be described generallyas unidirectional, in that it decreases fairly consistently from astrong signal to a very weak one, although it will be understood thatthe mean signal strength fluctuates somewhat and does not follow asmooth curve, sometimes even increasing slightly for a very shortperiod. In order to make a readable strip record it is readily apparentthat some sort of gain control in the amplifier is necessary, yet thatconventional automatic volume control circuits will fail to achieve thedesired results. Gain control which covers this range without distortingwave form or amplitude relation of adjacent oscillations has beenaccomplished in the amplifier illustrated herewith by combining anautomatic expander, which changes a portion of the gain of the amplifierwith respect to time independently of the signal, with a portion of theamplifier wherein automatic volume control means changes the gain by,but inversely to, the signal, in accordance with the average valuethereof. This automatic volume control portion comprises two parts, oneof which follows the signal envelope rather closely to generate therectified control voltage, the other serving as the main amplifyingchannel developing the output of the unit and being controlledsubstantially in accordance with the average amplitude of theoscillations by the voltage generated in the other portion.

In the particular embodiment of the invention illustrated herewith theinput portion of the amplifier, prior to the first tube, is shown inFig. 1. This comprises various relays, resistors, transformers, and thelike, making an operative input system capable of adjustment withrespect to the proportion of the input signal fed to the first tube, andan expander circuit adapted to vary a portion of the gain of theamplifier inversely to the signal variation during the same timeinterval, the operation of the expander being initiated by the "firstbreak" or initial input, but thereafter being independent of the signal.The portion illustrated in Fig. 1 comprises tubes arranged in cascade toprovide a main signal amplifying channel and other tubes arranged in acircuit operatively coupled to the main channel for achieving, from thevoltages of the signal envelope, a volume control which varies the mainchannel gain in accordance with but inversely to the mean or averagesignal amplitude.

Referring more particularly to Fig. 1, an input plug I is shown havingterminals l2, l3, I l, i6 and II, which terminals are adapted to haveleads from three different seismometers attached to |2, |3 and I4, thereturns being all through a common ground connection to the terminal II.The terminal I6 is supplied with a potential which, in this particularcase, is six volts negative with respect to chassis ground. This voltageserves to render an expander, hereinafter to be described, operative.

The group of switches indicated in general as l8 are adapted to bemanually operated for test purposes, and will not be further described.The switches in the group indicated in general as H! are for the purposeof achieving the necessary switching to, in. connection with the threetransformers 20, 2| and 22, place the input circuit in condition toenable three seismometer circuits to be connected together to form onetrace, to have only one seismometer circuit providing input energy tothe amplifier, or in combination with the relay 23, to provide energyfor a period from one seismometer circuit, and then for the remainder ofthe period from three. When the relays I9 are in the positionillustrated in the diagram, input from only one seismometer circuit issupplied to the amplifier until the group of relays 23 has shifted,which occurs a predetermined time interval after the first break, andthe input energy is then taken from three such circuits.

All of the movable armatures of the relay group 23 are actuated by theone relay coil 24 so that they all swing at once in response toenergization thereof. Means for energizing this actuating coil upon theinitial input signal or first break when a charge is fired may beaccomplished by any desired means, not here shown. Such means should,however, be unresponsive to transient voltage variations prior to thedetonation, and is preferably arranged so that there is some slight timeinterval after the detonation before the operation of the relay. Thegroup of switches 23 forming the relay is here shown in the position inwhich they would be before detonation of the charge; after the initialinput energizes the relay all of the movable armature members thereofwould swing to the left, speaking with respect to their positions asillustrated, and held there for the duration of the signal.

In order to reduce the first break input voltage to the tubes prior tooperation of the relay, a

resistor assembly 25 is employed. In the particular arrangementillustrated here any one of the resistors, which vary in value fromabout megohms down to about 150 ohms, may be placed in series with theseismometer input circuit, but they are adapted to be shorted out uponopera- 5 tion of the relay 23 by closing of the contacts of the switch23'. It will be understood that the resistor assembly should be arrangedeither in series or parallel with the seismometer cables in accordancewith whether the impedance char- ]0 acteristics are inductive orcapacitative, in order to keep cross feed or coupling between the cablesas low as possible.

The use of three transformers 2!], 2| and 22 for the input with separateload resistors 26, 2l 1:, and connected across the secondaries thereofpermits the addition of voltages across these resistor loads withoutundesired coupling between the seismometer circuits. These loadresistors, moreover, are of a variable type as shown 20 adapted topermit manual attenuation of the input signal to insure the desiredoutput from the amplifier. The fact that this main attenuator is locatedahead of the first tube of the main amplifier channel enablesoverloading of this first stage to be prevented. Any desired portion ofthe alternating voltage developed across the load resistors is takenthrough the wire or lead 29 to the grid element of the first amplifiertube.

One of the novel and particularly important portions of the amplifierdisclosed herewith is the expander circuit comprising the condenser 30,resistors 3|, 32 and 33, and the switch elements 23 Resistors 32 and 33are so chosen that their ratio is as the ratio of the desired residualor terminal grid voltage to the voltage applied through the terminal l6.In the present case the desired terminal voltage is one volt negative, afi-volt negative potential is applied to the terminal I6, and theresistors have a one to five ratio, the resistor 32 being 40,000 ohmsand the -resistor 33 having a value of .2 megohm, for example. It willbe noted that the resistor 33 is a potentiometer, the potential at anypoint thereof being applied through the wire 34, the switch elements 23and the wires 35 and 36 to the upper side of the condenser 30 and to thesecondary of one of the transformers, at 2|. Inasmuch as the other sideof the condenser 30 is connected to the wire 31, which wire forms thechassis ground,

the potential existing across the condenser, depending upon the settingof the variable resistor 33, is applied between the cathode and grid ofthe first amplifier tube to determine its initial sensitivity. As longas the circuit is complete through the switch points 23 the condenser 30remains at the chosen potential. As soon as this circuit is broken uponoperation of the relay 23, however, the condenser starts to dischargethrough the resistors 3| and 32, and thus the bias voltage applied by itto the grid decreases toward chassis ground potential. The time intervaltaken for the drop in voltage from the initial bias voltage toessentially the terminal bias voltage determined by the resistor 32,with any given initial voltage, is a function of the size of thecondenser 30 and the value of the resistors 3| and 32 through which itmust discharge. The condenser 30 may, for example, 7 be .25 micro-faradsand the resistor 3| one megohm. The value of the resistor 32 issubstantially negligible in such a case, and the discharge timeefl'ective to vary the gain of the tube 38 may be considered as afunction of the resistor 3|. In the case shown the expansion galactictime in seconds is approximately twice the value of the resistor it inmegohms. With the resistor shown, the voltage across the condenser M,applied to the grid of the first tube as a bias, would vary from itsinitial to essentially its terminal value in about two seconds.

Referring more particularly to Fig. 1, it will be seen that the first orinitial tube at is a high gain pentode tube of the variable mu type.Where the amplifier is designed for use as a battery operated unit thistubemay, for example, be of the 6K7 type. The characteristics of thistube are such that a -volt change in grid bias is sumcient to vary theamplification factor of the tube considerably. In the particular circuitillustrated the expander will, if the initial sensitivity control 33 isset to apply all of the available negative bias to the grid at thebeginning of th expansion period, vary the amplification factor of thetube 3t about thirty-four to one in the 2-second interval.

It will be seen that once action of the expander, varying theamplification factor with respect to time, is initiated theamplification factor of the tube will rise from its initial value to oneabout 34 times that during the interval. Once this change has beenstarted by operation of the relay 233 it is independent of the signal.That is, the rate of change is determined by the discharge of thecondenser through the resistor M, which is a function of time, and isnot dependent in any way upon the rate of change of the average signalor any individual oscillation thereof during the time interval. It willbe understood, however, that when the expander action is spoken of asbeing independent of the signal it is merely meant that the rate ofchange is a fixed function of the condenser and resistor, sinceinitiation of the expander action is dependent upon the first break orinitial signal, and the terminal voltage may, if desired, also be made afunction of the signal. The time interval through which it is desired tohave the amplifier operate to produce a record, in conjunction with acamera oscillograph, depends upon the terrain in which the charge isdetonated. The time interval from first break to the final reflectionwhich is of any value may run from one second up to about four seconds,but is generally between two to three seconds. The rate of change of theexpander voltage with respect to time should be set for approximatelythe same interval by changing the value of the resistor 30.

The use of such an expander, changing the amplification of at least onetube with respect to time, is of considerable advantage, particularlywhere it is coupled with automatic volume control in later portions ofthe amplifier. It has been found that too great automatic volume controlresults in many difficulties, both from the standpoint of poor recordsand of circuit instability. The use of an expander of this type ahead ofthe automatic volume control portion of the circuit, however, enablesthe automatic volume control system to carry a much smaller portion ofthe burden. In the present case, for example, the automatic volumecontrol need only have a maximum change in gain of one-thousand to lidone to give the amplifier an over-all variable gain\ Referring moreparticularly to the portion of the device comprising the electronicamplifying tubes, it will be seen that there is a main amplifier channelcomprising the tube 38, the two tubes 39 and 40 arranged in push-pullrelation, and the output tube 4! feeding into the output couplingtransformer 42 leading to the camera oscillograp'h. The transformer 43handling the output of the driver tube 38 has two secondaries, one beingconnected to the tubes 39 and M and the other supplying energy to thegrid elements of another pair of tubes H and is arranged for push-pulloperation. The output of these latter tubes is fed to the tube 46, andthese three last mentioned tubes serve as an auxiliary system providingautomatic volume control bias to achieve the desired control of theover-all gain of the push-pullstage in the main channel of theamplifier. The tubes 39, to, M and 45 are also here shown as of the 6K7type, while the tube 46 is a duo-diode-triode tube of the SQ type. Thetubes in each of these push-pull stages must be of similarcharacteristics and each pair, must be carefully balanced, so thatchanges in grid or control bias will be neutralized and will not causesuch unbalancing of plate current as to be reflected in the record as asignal. The transformers and other circuit elements should be soselected that the frequency response in the auxiliary or AVC portion ofthe amplifier is nearly the same as that in the main amplifying channel,having preferably a slightly higher cut-off.

The various tubes and associated portions of the circuit are supplied inconventional manner with the necessary voltages to render the deviceoperative, usually by batteries when the device is being used in theheld for geophysical work.

The screen grids are preferably supplied with voltage through a separatebattery connected to the lead wire ll, rather than by a portion of theplate voltage supply, since small changes in screen grid voltage intubes of this type will cause a change in the amplification factor.

The impedance of the grid circuits of the pushpull stages is made highby the use of resistors 48, d9, 50 and M, to prevent the surge at thefirst break, when the sensitivity and gain of the main portion of theamplifier is high and the grids of the push-pull stages are swungpositive, from permitting enough current to flow into the condensers 52and 53 to cause them to block the tubes for an undesired intervalthereafter. These resistors may all, for example, have a value of onemegohm.

If too large an amount of energy is fed in to the push-pull stages for afew tenths of a second by the early signal the AVG section of theamplifier will build up so much control voltage that it reduces the gainof the amplifier to such a point that weak signals following the seriesof first strong impulses will be of too small magnitude. This conditionis a result of the accommodation or over-all change of gain ratio of theamplifier being too great, so that the control voltage becomesdisproportionately large with respect to the signal input. This isundesirable, and the initial sensitivity setting should be changed sothat more accommodation will be taken care of by the expander, or theseries re- :isltor increased to cut down th input to the first Referringmore particularly to the auxiliary amplifier for the automatic volumecontrol voltage, it will be seen that the tubes 44 and 4-5 ieed into thetransformer it having the load resistor 55 across the secondary thereof,the voltage developed across this resistor being applied between thecathode and grid of the triode portion of the tube 46. The output of thetriode portion is developed across the transformer I which has itssecondary connected to the diode elements of the tube to achieve fullwave rectification. Since the cathode for the diodes is not directlyconnected to chassis ground, but rather through the resistor 56 and 51,a delay action is achieved. These resistors should be so chosen as tomake the cathode about three volts positive with respect to ground, sothat there is no rectification until the'voltage across the secondary ofthe transformer In exceeds six volts. The rectified control voltage,developed across the resistor 58, is applied through a small battery (toinsure the desired initial bias on the tubes) and the filter resistor 80through the resistors 50 and 5| to the control grids of the tubes 44 and45 respectively, in the auxiliary portion of the amplifier. It will benoted that the lead 6| which connect the filter resistor 60 to theresistors 50 and 5| is connected to chassis ground 31 through thecondenser 53. Before any appreciable voltage is applied to the grids ofthe tubes 44 and 45, therefore, the condenser 53 must be charged to thatvoltage. The time constant of the auxiliary portion of the circuit isthus a function of the filter resistor 60 and of the condenser 53.

The total available grid bias voltage, developed across both theresistors 58 and 59, is transmitted through the filter resistor 62 andthe lead 63 to the resistors 48 and 49, through which it is applied tothe grids of the tubes 38 and 40 in the main amplifier channel. The lead83 is by-passed to chassis ground by the condenser 52, and the timeconstant or the AVG action in the main channel of the amplifier is thusa function of the resistor 62 and the condenser 52.

In the particular embodiment here shown the resistor 62 is chosen withthe value of one megohm and the resistor 60 with the value of onehaif amegohm; while the condenser 52 has a value of .l micro-fared and thecondenser 53 of .05 micro-farad. The time constant of the control actionin the main channel of the amplifier is about one-tenth of a second, orslightly higher, and the time constant of the automatic volume controlor auxiliary portion of the amplifier is about one-fourth of that. Theshorter time constant in the auxiliary section of the amplifier enablesit to rapidly follow the envelope of the reflection waves in the input,so that the diode' voltage truly represents an average value of theproper voltage for a constant average output, dependent upon the meansignal input existing at the secondary of the transformer 43. The timeconstant in the main channel of the amplifier is sufficiently longenough, however, that individual fluctuations are not followed, but instead there is a relatively slow change in bias approximating theaverage change of the input to the push-pull stages, so that thedecrease in average signal strength during the interval of thereflections is compensated for, yet outstanding single oscillations,whether they be strong or weak, are handled by the main amplifierwithout distorting their magnitude with respect to other closelyadjacent reflections, or other portions of the same reflection. That is,a strong initial alternation at the beginning of a reflection, fol- Gillowed by a much weaker alternation, is faithfully reproduced on therecord, while at the same time there is a slow change of amplificationfactor or gain of the push-pull stage of the main channel so thatreflections near the end of the interval are amplified sufilciently tobe comparable with those first appearing on the record, and easily read.The time constant of the circuit supplying bias voltage to the auxiliaryamplifier, to achieve the desired results, should be from to that of thecircuit supplying the bias voltage to the main amplifier, and preferablyabout The main channel, since it gets more control voltage than theauxiliary channel, can be held to a very nearly constant output; at thesame time, the fact that the auxiliary or automatic volume controlchannel receives the energy from the input to the push-pull stage in themain channel, and is capable of some swing in amplification factor,enables a very true straight line output to be achieved in the mainchannel by closely following the signal envelope. In fact, the value ofthe resistor 59 can be increased to the point where a rise in input tothe amplifier will actually result in a decrease in output, so that itwill be seen that the output may be controlled ln any desired manner.Increasing the time constant or time lag too much in the application ofthe automatic volume control bias to the main channel gives pooramplitude control, in that the change in amplification factor may lagtoo far behind changes in input. If the time constant is too short, onthe other hand, the later legs or alternations of one reflection may bereduced as compared to the first strong legs, so that a record will behad which does not reproduce true conditions.

When it is desired to take-a record of a particular detonation theamplifier is first tested to make sure that all of its parts are inproper working order, and to adjust the main attenuator and othercircuit variables so that the expected signal input and time intervalwill be compensated for. This includes adjustment of the resistor 25 inaccordance with the expected power of the first impulses; of the mainattenuator comprising the portions 26, 21 and 28 in accordance with theexpected power of the final impulses; of the variable resistor 33 forthe initial sensitivity desired; the choice of the resistor 3| for thetime interval expected or found by previous experiments to exist in thatlocality; and sometimes adjustment of the resistor 64. Proper choice ofthe resistor 31 enables the expander action to cover substantially thesame time interval as the expected variations in input.

When the first impulse reaches the amplifier the tube 38 has a ratherlow amplification factor, determined by the position of the initialsensitivity setting, and a load resistor of the group 25 is in thecircuit. Since there is no signal passing through the amplifier,however, the

automatic volume control is inactive and the gain of the push-pull stagein the main channel is rather high, so that a considerable amount ofenergy is capable of being developed in the output. In order to preventthis from damaging the galvanometer employed to make the record,generally of the string type, a deflection limiter is included in thecircuit. This deflection limiter is shown as comprising a pair ofrectifiers 65 and 66 connected back to back in conjunction with aresistor 61. These rectifiers are biased by the aciacae voltagedeveloped in resistor ti so that the input must exceed the 'biasvoltage, below that which would damage the galvanometer, beforerectification occurs, but when the peaks of the first few alternationspass this break down voltage which is substantially a short circuit isplaced on the output. The deflection limiter is in series with theswitch elements 23 of the relay 2i. however, and as soon as the relayoperates the defiection limiter is cut out of circuit. This is generallyabout one-tenth of a second, and in this time or less the automaticvolume control has reached equilibrium and is capable of controllingthis input intensity. Upon operation of the relay other events takeplace, including cutting out the particular resistor in the assembly 25in series with the input by the switch elements tt and initiation ofexpander action by the switch elements M That is, when these latterelements breair, the condenser starts to discharge to increase theamplification factor of the tube it.

An amplifier is thus provided particularly adapted for work of the typewhere there is no carrier, and control of a wide variation in gain ofthe amplifier must be achieved by the signal. At the same time, thesystem is so arranged that good quality is retained, the'wave form isnot distorted. and the relative magnitudes of adjacent alternations orlegs of a reflection are reproduced in comparable magnitude upon therecord.

While I have described and claimed certain embodiments of my inventionit is to be understood that it is capable of many modifications.changes, therefore, in the construction and arrangement may be madewithout departing from the spirit and scope of the invention asdisclosed in the appended claims in which it is my intention to claimall novelty inherent in my invention as broadly as permissible in viewof the prior art.

I claim:

1. An electronic amplifier of the character described adapted tomaintain a substantially constant output when the mean signal inputthereto varies substantially uni-directionally through a wide rangeduring an interval of time, including: an amplifier having at least twotubes in cascade; means independent of said signal for automaticallyvarying the gain of at least one tube inversely to said input duringsaid interval; and means for varying the gain of at least one succeedingtube by but inversely to said signal variation.

2. An electronic amplifier of the character described adapted tomaintain a substantially constant output when the mean signal inputthereto varies substantially uni-directionally through a wide rangeduring an interval of time, including: an amplifier having at least twotubes in cascade; means independent of said signal for automaticallyvarying the gain of at least one tube inversely to said input duringsaid interval; means for rendering said automatically varying meansoperable substantially simultaneously with initiation of said inputvariation; and means for varying the gain of at least one succeedingtube by said mean signal variation but inversely thereto.

3. An electronic amplifier of the character described adapted tomaintain a substantially constant output when the mean signal inputthereto varies substantially uni-directionally through a wide rangeduring a short interval of time, including: a plurality of amplifyingtubes in cascade, at least two of said tubes having variableamplification factors; mean for impressing a voltage on an element ofone or said tubes havin a variable amplification factor to control theamplification factor thereof; means for varying said voltage saidvariation being a function of time only, said last mentioned means beingarranged to effect a variation in said amplification factor inverse tosaid variation of the input and coincident therewith; meansforimpressing avoltage on an element of the other of said tubes having avariable amplification factor to control the amplification factorthereof, this tube succeeding the first mentioned tube with a variableamplification factor; and means for varying said lastmentioned voltageby said signal variation but inversely thereto.

4. Apparatus of the character described for amplifying an oscillatingsignal and maintaining a substantially constant mean output when themean signal input thereto varies widely, including: a main amplifieradapted to furnish said output, said amplifier having a variable gain;an auxiliary amplifier having a variable gain; means for rectifying theoutput of said auxiliary amplifier to provide a control voltage; and acircuit applying said control voltage to said amplifiers to vary thegain thereof, the time constant of the application to said mainamplifier being several times that to said auxiliary amplifier.

5. Apparatus of the character described for amplifying an oscillatingsignal and-maintaining a substantially constant mean output when themean signal input thereto varie widely, including: a main amplifieradapted to furnish said output, said amplifier having a variable gainadapted to be controlled by a bias voltage; an auxiliary amplifierhaving a variable gain adapted to be controlled by a bias voltage; meansfor rectifying the output of said auxiliary amplifier to provide a biasvoltage; a circuit applying said bias voltage to said main amplifier,said circuit in cluding a resistance element and a condenser element;and a second circuit applying at least a portion of said bias voltage tosaid auxiliary amplifier, said circuit also including a resistanceelement and a condenser element, at least one of said elements having alesser value than the corresponding element in said first circuit.

6. Apparatus of the character described for amplifying an oscillatingsignal and maintaining a substantially constant mean output when themean signal input thereto varies widely, including: a main amplifyingchannel adapted to furnish said output, said channel comprising aplurality of tubes arranged in cascade, each of said tubes having acathode, grid, and plate element and at least one of said tubes havingan amplification factor variable in accordance with a bias voltageapplied between the cathode and the grid thereof; an auxiliaryamplifying channel, said channel comprising at least one tube having acathode, grid, and plate element and having an amplification factorvariable in accordance with a bias voltage applied between the cathodeand grid thereof; means for diverting a portion of said signal to saidauxiliary amplifier from said main channel prior to at least one of thetubes therein having a variable amplification factor; means forrectifying the output of said auxiliary amplifier to provide a biasvoltage; a circuit applying said bias voltage to at least one of thetubes in said main channel subsequent to said diversion, s'aid circuitincluding a condenser in shunt between the cathode and grid of said tubeand a resistance in series with one of said tube elements; and asecondcircuit applying the bias voltage to the tube elements of saidauxiliary amplifying channel, said second circuit including a condenserand resistance similarly arranged but having values such that the timeconstant of said second circuit is about one-fifth that of said firstcircuit.

7. Apparatus of the character claimed in claim 6, wherein theoscillating signal has a frequency of less than 100 cycles per secondand the first circuit has a time constant of about one-tenth oi asecond. a

8. Apparatus of the character described for amplifying an oscillatingsignal of a low frequency and for maintaining a substantially constantmean output when the mean signal input thereto varies widely, including:a plurality of electronic tubes arranged in cascade for ampli- :fyingsaid signal to furnish said output, at least two of said tubes having avariable amplification factor; auxiliary amplifying means; means fordiverting a portion of said signal to said second amplifier; means forrectifying the output of said auxiliary amplifier to furnish a signalcontrolled voltage for varying the amplification factor of a tube insaid cascade of tubes; a second source of bias voltage adapted tocontrol the amplification factor of a tube in said cascade earlier thanthat to which said signal controlled voltage is applied; and means forautomatically varying said bias voltage with respect to time andindependently of said signal.

9. Apparatus of the character claimed in claim 8. wherein said signal isdiverted between said time controlled and said signal controlled tubesand the variation in gain of the time controlled portion of saidamplifier is less than that of the signal controlled portion.

10. Apparatus for controlling the amplification of signals varying inaverage amplitude in ac-- cordance with time, which comprises anamplifier for said signals, gain control means for varying theamplification of said signals as a function of time, and monitoringmeans operating upon the amplified signals which have been subjected tosaid gain control means for further controlling the amplification as afunction of the strength of said last mentioned signals, whereby outputsignals of substantially constant average amplitude are obtained.

11. Apparatus for controlling the amplification of signals of graduallydecreasing amp itude, which comprises an amplifier for said signals,gain control means for increasing the amplitude of the amplified signalsin rough accordance with the gradually decreasing amplitude of theoriginal signals, whereby signals having decreased variation of averageamplitude are obtained, and monitoring means operating upon theamplified signals which have been subjected to said gain control meansfor further controlling the amplification as a function of the strengthof said last mentioned signals, whereby output signals of substantiallyconstant average amplitude are obtained.

12. An amplifier system for use in seismic surveying which comprises anamplifier operating upon signals varying in average amplitude inaccordance with the average amplitude of seismic waves of graduallydecreasing average amplitude, gain control means for increasing theamplification of the signals in rough accordance with the graduallydecreasing amplitude of the original signals, whereby signals havingdecreased variation of average amplitude are obtained, and monitoringmeans operating upon the amplified signals which have been subjected tosaid gain control means for further controlling the amplification as afunction of the strength of said last mentioned signals, whereby outputsignals of substantially constant average amplitude are obtained.

LE ROY C. PASLAY.

